The hypertensive response to the infusion of hemoglobin has been attributed to the reaction between hemoglobin and the endothelial derived nitric oxide (NO). The pro or anti-oxidant properties of NO depend on the chemistry it undergoes in a biological system, and that under certain conditions the balance between NO/superoxide ion(O.-2)may be channeled by way of peroxynitrite (ONOO), a byproduct of NO reactions with superoxide ion. Reactions of native and chemically modified hemoglobins with ONOO lead to rapid oxidation of the heme iron to the ferric form and possibly other oxidation products. Time courses obtained from rapid kinetic experiments showed a biphasic loss of the ferrous oxidation state on mixing ONOO with these hemoglobins. Using colorimetric and chemiluminescent immunodetection techniques, we were able to show that tyrosine nitration by ONOO; can indeed occur with both native and modified hemoglobins. A mechanism describing the reaction of ONOO; with a number of chemically modified hemoglobins has recently been published in Arch. Biochem. Biophysics (349:65-73, 1998). We have recently measured the levels of glutathione (GSH), a marker of oxidative stress, in cultured bovine aortic endothelial cell (BAECs) and determined the effects of extracellular hemoproteins on programmed cell death in the presence of physiological oxidants. Exogenous H2O2 and ONOO produced concentration-and time-dependent apopotosis in BAECs as measured by annexin V-binding assay and DNA fragmentation. In the presence of myoglobin or hemoglobin, both H2O2 and ONOO; produced a marked drop in GSH compared to control, oxidant alone, or the hemoproteins. Moreover, in the presence of hemoproteins a necrotic (non programmed) pattern of cell death appeared to predominate over apoptosis (programmed). In a collaborative study with NIDDK, NIH we have recently started to explore the mechanism of nitric oxide-mediated allosteric effects on the oxygen affinity of red cells from patients with sickle cell anemia and the potential clinical benefits of treating this disease with NO-derived products. Data so far indicate that a selective nitrosylation of the b-chains of hemoglobin by nitric oxide donor compounds is responsible for the observed alteration in the oxygen affinity of the intra-erythrocytic hemoglobin with no significant protein oxidative modification. NO also decreases platelet activation via a cGMP-depnedent mechanism. We have recently examined the effects of a number of NO donors on platelet activation from sickle cell patients. We found that the method of delivery of NO to platelets has varying effects upon platelet responses, and that the utilization of a more effective delivey sytem may utlimatly contribute to reducing platelet activation in patients with sickle cell diesese.